Activation of herpesvirus gene expression by the human cytomegalovirus protein pp71

Epstein-Barr Virus (EBV) Tegument Protein BGLF2 Promotes EBV Reactivation through Activation of the p38 Mitogen-Activated Protein Kinase

Epstein-Barr virus (EBV) is a ubiquitous gammaherpesvirus associated with both B cell and epithelial cell malignancies. EBV infection of B cells triggers activation of several signaling pathways that are critical for cell survival, virus latency, and growth transformation. To identify EBV proteins important for regulating cell signaling, we used a proteomic approach to screen viral proteins for AP-1 and NF-κB promoter activity in AP-1- and NF-κB-luciferase reporter assays. We found that EBV BGLF2 activated AP-1 but not NF-κB reporter activity. Expression of EBV BGLF2 in cells activated p38 and c-Jun N-terminal kinase (JNK), both of which are important for mitogen-activated protein kinase (MAPK) signaling. Deletion of the carboxyl-terminal 66 amino acids of BGLF2 reduced the ability of BGLF2 to activate JNK and p38. Expression of BGLF2 enhanced BZLF1 expression in latently EBV-infected lymphoblastoid cell lines, and knockdown of BGLF2 reduced EBV reactivation induced by IgG cross-linking. Expression of BGLF2 induced BZLF1 expression and virus production in EBV-infected gastric carcinoma cells. BGLF2 enhanced BZLF1 expression and EBV production by activating p38; chemical inhibition of p38 and MAPK/ERK kinases 1 and 2 (MEK1/2) reduced expression of BZLF1 and virus production induced by BGLF2. In summary, the EBV tegument protein BGLF2, which is delivered to the cell at the onset of virus infection, activates the AP-1 pathway and enhances EBV reactivation and virus production.

IMPORTANCE

Epstein-Barr virus (EBV) is associated with both B cell and epithelial cell malignancies, and the virus activates multiple signaling pathways important for its persistence in latently infected cells. We identified a viral tegument protein, BGLF2, which activates members of the mitogen-activated protein kinase signaling pathway. Expression of BGLF2 increased expression of EBV BZLF1, which activates a switch from latent to lytic virus infection, and increased production of EBV. Inhibition of BGFL2 expression or inhibition of p38/MAPK, which is activated by BGLF2, reduced virus reactivation from latency. These results indicate that a viral tegument protein which is delivered to cells upon infection activates signaling pathways to enhance virus production and facilitate virus reactivation from latency.

The human cytomegalovirus UL133-138 gene locus attenuates the lytic viral cycle in fibroblasts

The genomes of HCMV clinical strains (e.g. FIX, TR, PH, etc) contain a 15 kb region that encodes 20 putative ORFs. The region, termed ULb’, is lost after serial passage of virus in human foreskin fibroblast (HFF) cell culture. Compared to clinical strains, laboratory strains replicate faster and to higher titers of infectious virus. We made recombinant viruses with 22, 14, or 7 ORFs deleted from the ULb’ region using FIX and TR as model clinical strains. We also introduced a stop codon into single ORFs between UL133 and UL138 to prevent protein expression. All deletions within ULb’ and all stop codon mutants within the UL133 to UL138 region increased to varying degrees, viral major immediate early RNA and protein, DNA, and cell-free infectious virus compared to the wild type viruses. The wild type viral proteins slowed down the viral replication process along with cell-free infectious virus release from human fibroblast cells.

Herpes simplex viral-vector design for efficient transduction of nonneuronal cells without cytotoxicity

The design of highly defective herpes simplex virus (HSV) vectors for transgene expression in nonneuronal cells in the absence of toxic viral-gene activity has been elusive. Here, we report that elements of the latency locus protect a nonviral promoter against silencing in primary human cells in the absence of any viral-gene expression. We identified a CTCF motif cluster 5' to the latency promoter and a known long-term regulatory region as important elements for vigorous transgene expression from a vector that is functionally deleted for all five immediate-early genes and the 15-kb internal repeat region. We inserted a 16.5-kb expression cassette for full-length mouse dystrophin and report robust and durable expression in dystrophin-deficient muscle cells in vitro. Given the broad cell tropism of HSV, our design provides a nontoxic vector that can accommodate large transgene constructs for transduction of a wide variety of cells without vector integration, thereby filling an important void in the current arsenal of gene-therapy vectors.

Immediate-Early (IE) gene regulation of cytomegalovirus: IE1- and pp71-mediated viral strategies against cellular defenses

Three crucial hurdles hinder studies on human cytomegalovirus (HCMV): strict species specificity, differences between in vivo and in vitro infection, and the complexity of gene regulation. Ever since the sequencing of the whole genome was first accomplished, functional studies on individual genes have been the mainstream in the CMV field. Gene regulation has therefore been elucidated in a more detailed fashion. However, viral gene regulation is largely controlled by both cellular and viral components. In other words, viral gene expression is determined by the virus-host interaction. Generally, cells respond to viral infection in a defensive pattern; at the same time, viruses try to counteract the cellular defense or else hide in the host (latency). Viruses evolve effective strategies against cellular defense in order to achieve replicative success. Whether or not they are successful, cellular defenses remain in the whole viral replication cycle: entry, immediate-early (IE) gene expression, early gene expression, DNA replication, late gene expression, and viral egress. Many viral strategies against cellular defense, and which occur in the immediate-early time of viral infection, have been documented. In this review, we will summarize the documented biological functions of IE1 and pp71 proteins, especially with regard to how they counteract cellular intrinsic defenses.

Functional annotation of human cytomegalovirus gene products: an update

Human cytomegalovirus is an opportunistic double-stranded DNA virus with one of the largest viral genomes known. The 235 kB genome is divided in a unique long (UL) and a unique short (US) region which are flanked by terminal and internal repeats. The expression of HCMV genes is highly complex and involves the production of protein coding transcripts, polyadenylated long non-coding RNAs, polyadenylated anti-sense transcripts and a variety of non-polyadenylated RNAs such as microRNAs. Although the function of many of these transcripts is unknown, they are suggested to play a direct or regulatory role in the delicately orchestrated processes that ensure HCMV replication and life-long persistence. This review focuses on annotating the complete viral genome based on three sources of information. First, previous reviews were used as a template for the functional keywords to ensure continuity; second, the Uniprot database was used to further enrich the functional database; and finally, the literature was manually curated for novel functions of HCMV gene products. Novel discoveries were discussed in light of the viral life cycle. This functional annotation highlights still poorly understood regions of the genome but more importantly it can give insight in functional clusters and/or may be helpful in the analysis of future transcriptomics and proteomics studies.

Highly acidic C-terminal region of cytomegalovirus pUL96 determines its functions during virus maturation independently of a direct pp150 interaction

Tegument proteins pp150 and pUL96 function at a late step in cytomegalovirus (CMV) maturation. Here, we show that pp150 interacts directly with pUL96; however, the N-terminal region of pp150 and the C-terminal region of pUL96, which are critical for these proteins to function, are not required for this interaction. Moreover, the largely dispensable C-terminal region of pp150 is critical for pp150-pUL96 interaction. To further study the role of pUL96, several point and clustered mutations were engineered into the CMV Towne bacterial artificial chromosome (Towne-BAC) genome, replacing the conserved negatively charged C-terminal residues of pUL96. Although individual point mutations (E122A, D124A, and D125A) reduced virus growth slightly, the clustered mutations of 122EVDDAV127 significantly reduced virus growth, produced small syncytial plaque phenotypes, and impacted a late stage of virus maturation. When the UL96 C-terminal alanine conversion mutant (B6-BAC) virus was serially passaged in cell culture, it gained a plaque size comparable to that of Towne-BAC, displayed an altered restriction fragment length pattern, and replicated with increased growth kinetics. Whole-genome sequencing of this passaged virus (UL96P10) and the similarly passaged Towne-BAC virus revealed major differences only in the RNA4.9 and UL96 regions. When one of the mutations in the UL96 coding region was engineered into the B6-BAC virus, it significantly increased the plaque size and rescued the virus growth rate. Thus, accumulation of compensatory mutations only in UL96 in this revertant and the specific involvement of functionally dispensable regions of pp150 in the pUL96-pp150 interaction point toward a role for pUL96 in virus maturation that does not depend upon pp150.

IMPORTANCE

Human cytomegalovirus causes significant medical problems in newborns, as well as in people with low immunity. In this study, we investigated the functions of two essential virus proteins, pp150 and pUL96, and determined the impact of their mutual interaction on virus replication. These studies provide valuable information that is critical for the development of targeted antiviral therapies.

Virion factors that target Daxx to overcome intrinsic immunity

PML nuclear bodies and their associated functions are part of an intrinsic cellular mechanism aimed at maintaining transcriptional control over viral gene expression and preventing replication of invading viruses. To overcome these barriers, many viruses express early nonstructural, multifunctional proteins to support the viral replication cycle or modulate host immune responses. Virion proteins constituting the invading particle are traditionally investigated for their role in transport during entry or egress and in the assembly of new virions. The additional functions of virion proteins have largely been ignored, in contrast to those of their nonstructural counterparts. A number of recent reports suggest that several virion proteins may also play vital roles in gene activation processes, in particular by counteracting intrinsic immune mechanisms mediated by the PML nuclear body-associated cellular factors Daxx, ATRX, and Sp100. These virion proteins share several features with their more potent nonstructural counterparts, and they may serve to bridge the gap in the early phase of an infection until immediate early viral gene expression is established. In this review, we discuss how virion proteins are an integral part of gene regulation among several viral families and to what extent structural proteins of incoming virions may contribute to species barrier, latency, and oncogenesis.

The replication defect of ICP0-null mutant herpes simplex virus 1 can be largely complemented by the combined activities of human cytomegalovirus proteins IE1 and pp71

Herpes simplex virus 1 (HSV-1) immediate-early protein ICP0 is required for efficient lytic infection and productive reactivation from latency and induces derepression of quiescent viral genomes. Despite being unrelated at the sequence level, ICP0 and human cytomegalovirus proteins IE1 and pp71 share some functional similarities in their abilities to counteract antiviral restriction mediated by components of cellular nuclear structures known as ND10. To investigate the extent to which IE1 and pp71 might substitute for ICP0, cell lines were developed that express either IE1 or pp71, or both together, in an inducible manner. We found that pp71 dissociated the hDaxx-ATRX complex and inhibited accumulation of these proteins at sites juxtaposed to HSV-1 genomes but had no effect on the promyelocytic leukemia protein (PML) or Sp100. IE1 caused loss of the small ubiquitin-like modifier (SUMO)-conjugated forms of PML and Sp100 and inhibited the recruitment of these proteins to HSV-1 genome foci but had little effect on hDaxx or ATRX in these assays. Both IE1 and pp71 stimulated ICP0-null mutant plaque formation, but neither to the extent achieved by ICP0. The combination of IE1 and pp71, however, inhibited recruitment of all ND10 proteins to viral genome foci, stimulated ICP0-null mutant HSV-1 plaque formation to near wild-type levels, and efficiently induced derepression of quiescent HSV-1 genomes. These results suggest that ND10-related intrinsic resistance results from the additive effects of several ND10 components and that the effects of IE1 and pp71 on subsets of these components combine to mirror the overall activities of ICP0.

Coordination of late gene transcription of human cytomegalovirus with viral DNA synthesis: recombinant viruses as potential therapeutic vaccine candidates

INTRODUCTION

During productive infection, human cytomegalovirus (HCMV) genes are expressed in a temporal cascade, with temporal phases designated as immediate-early (IE), early, and late. The major IE (MIE) genes, UL123 and UL122 (IE1/IE2), play a critical role in subsequent viral gene expression and the efficiency of viral replication. The early viral genes encode proteins necessary for viral DNA replication. Following viral DNA replication, delayed-early and late viral genes are expressed which encode structural proteins for the virion. The late genes can be divided into two broad classes. At early times the gamma-1 or leaky-late class are expressed at low levels after infection and are dramatically upregulated at late times. In contrast, the gamma-2 or 'true' late genes are expressed exclusively after viral DNA replication. Expression of true late (gamma-2 class) viral genes is completely prevented by inhibition of viral DNA synthesis.

AREAS COVERED

This review addresses the viral genes required for HCMV late gene transcription. Recombinant viruses that are defective for late gene transcription allow for early viral gene expression and viral DNA synthesis, but not infectious virus production. Since current HCMV prophylaxis is limited by several shortcomings, the use of defective recombinant viruses to induce HCMV cell-mediated and humoral immunity is discussed.

EXPERT OPINION

HCMV DNA replication and late gene transcription are not completely linked. Viral-encoded trans-acting factors are required. Recombinant viruses proficient in MIE and early viral gene expression and defective in late gene expression may be an alternative therapeutic vaccine candidates for the induction of cell-mediated and humoral immunity.

Tale of a tegument transactivator: the past, present and future of human CMV pp71

Herpesviruses assemble large virions capable of delivering to a newly infected cell not only the viral genome, but also viral proteins packaged within the tegument layer between the DNA-containing capsid and the lipid envelope. In this review, we describe the tegument transactivator of the β-herpesvirus human CMV, the pp71 protein. We present the known mechanistic features through which it activates viral gene expression during a lytic infection but fails to do so when the virus establishes latency, and describe how pp71 stimulates the cell cycle and may help infected cells avoid detection by the adaptive immune system. A historical overview of pp71 is extended with current perceptions of its roles during human CMV infections and suggestions for future avenues of experimentation.

An investigation of herpes simplex virus promoter activity compatible with latency establishment reveals VP16-independent activation of immediate-early promoters in sensory neurones

Herpes simplex virus (HSV) type-1 establishes lifelong latency in sensory neurones and it is widely assumed that latency is the consequence of a failure to initiate virus immediate-early (IE) gene expression. However, using a Cre reporter mouse system in conjunction with Cre-expressing HSV-1 recombinants we have previously shown that activation of the IE ICP0 promoter can precede latency establishment in at least 30% of latently infected cells. During productive infection of non-neuronal cells, IE promoter activation is largely dependent on the transactivator VP16 a late structural component of the virion. Of significance, VP16 has recently been shown to exhibit altered regulation in neurones; where its de novo synthesis is necessary for IE gene expression during both lytic infection and reactivation from latency. In the current study, we utilized the Cre reporter mouse model system to characterize the full extent of viral promoter activity compatible with cell survival and latency establishment. In contrast to the high frequency activation of representative IE promoters prior to latency establishment, cell marking using a virus recombinant expressing Cre under VP16 promoter control was very inefficient. Furthermore, infection of neuronal cultures with VP16 mutants reveals a strong VP16 requirement for IE promoter activity in non-neuronal cells, but not sensory neurones. We conclude that only IE promoter activation can efficiently precede latency establishment and that this activation is likely to occur through a VP16-independent mechanism.

Human cytomegalovirus immediate early gene expression in the osteosarcoma line U2OS is repressed by the cell protein ATRX

The control of human cytomegalovirus (HCMV) immediate early (IE) gene expression in infected human fibroblasts was compared with that in the U2OS human osteosarcoma cells. Viral IE expression was stimulated by the virion protein pp71 and repressed by the cell protein hDaxx in fibroblasts, as expected from published data. Neither of these events occurred in infected U2OS cells, suggesting that this cell line lacks one or more factors that repress HCMV IE expression. The chromatin remodeling factor ATRX is absent from U2OS cells, therefore the effect of introducing this protein by electroporation of plasmid DNA was investigated. Provision of ATRX inhibited HCMV IE expression, and the presence of the HCMV-specified virion phosphoprotein pp71 overcame the repression. The experiments demonstrate that ATRX can act as a cellular intrinsic antiviral defense in U2OS cells by blocking gene expression from incoming HCMV genomes. In contrast, ATRX did not affect the replication of herpes simplex virus type 1, showing that there are differences in the way U2OS cells respond to the presence of the herpesviral genomes.

Noncytotoxic inhibition of cytomegalovirus replication through NK cell protease granzyme M-mediated cleavage of viral phosphoprotein 71

Granzyme M (GrM) is highly expressed in cytotoxic granules of NK cells, which provide the first line of defense against viral pathogens. GrM knockout mice show increased susceptibility toward murine CMV infection. Although GrM is a potent inducer of cell death, the mechanism by which GrM eliminates viruses remains elusive. In this paper, we show that purified human GrM in combination with the perforin-analog streptolysin O (SLO) strongly inhibited human CMV (HCMV) replication in fibroblasts in the absence of host cell death. In a proteomic approach, GrM was highly specific toward the HCMV proteome and most efficiently cleaved phosphoprotein 71 (pp71), an HCMV tegument protein that is critical for viral replication. Cleavage of pp71 occurred when viral lysates were incubated with purified GrM, when intact cells expressing recombinant pp71 were challenged with living cytotoxic effector cells, and when HCMV-infected fibroblasts were incubated with SLO and purified GrM. GrM directly cleaved pp71 after Leu(439), which coincided with aberrant cellular localization of both pp71 cleavage fragments as determined by confocal immunofluorescence. In a luciferase reporter assay, cleavage of pp71 after Leu(439) by GrM completely abolished the ability of pp71 to transactivate the HCMV major immediate-early promoter, which is indispensable for effective HCMV replication. Finally, GrM decreased immediate-early 1 protein expression in HCMV-infected fibroblasts. These results indicate that the NK cell protease GrM mediates cell death-independent antiviral activity by direct cleavage of a viral substrate.

Chromatinisation of herpesvirus genomes

The double-stranded DNA genomes of herpesviruses exist in at least three alternative global chromatin states characterised by distinct nucleosome content. When encapsidated in virus particles, the viral DNA is devoid of any nucleosomes. In contrast, within latently infected nuclei herpesvirus genomes are believed to form regular nucleosomal structures resembling cellular chromatin. Finally, during productive infection nuclear viral DNA appears to adopt a state of intermediate chromatin formation with irregularly spaced nucleosomes. Nucleosome occupancy coupled with posttranslational histone modifications and other epigenetic marks may contribute significantly to the extent and timing of transcription from the viral genome and, consequently, to the outcome of infection. Recent research has provided first insights into the viral and cellular mechanisms that either maintain individual herpesvirus chromatin states or mediate transition between them. Here, we summarise and discuss both early work and new developments pointing towards common principles pertinent to the dynamic structure and epigenetic regulation of herpesvirus chromatin. Special emphasis is given to the emerging similarities in nucleosome assembly and disassembly processes on herpes simplex virus type 1 and human cytomegalovirus genomes over the course of the viral productive replication cycle and during the switch between latent and lytic infectious stages.

Properties of virion transactivator proteins encoded by primate cytomegaloviruses

Background

Human cytomegalovirus (HCMV) is a betaherpesvirus that causes severe disease in situations where the immune system is immature or compromised. HCMV immediate early (IE) gene expression is stimulated by the virion phosphoprotein pp71, encoded by open reading frame (ORF) UL82, and this transactivation activity is important for the efficient initiation of viral replication. It is currently recognized that pp71 acts to overcome cellular intrinsic defences that otherwise block viral IE gene expression, and that interactions of pp71 with the cell proteins Daxx and ATRX are important for this function. A further property of pp71 is the ability to enable prolonged gene expression from quiescent herpes simplex virus type 1 (HSV-1) genomes. Non-human primate cytomegaloviruses encode homologs of pp71, but there is currently no published information that addresses their effects on gene expression and modes of action.

Results

The UL82 homolog encoded by simian cytomegalovirus (SCMV), strain Colburn, was identified and cloned. This ORF, named S82, was cloned into an HSV-1 vector, as were those from baboon, rhesus monkey and chimpanzee cytomegaloviruses. The use of an HSV-1 vector enabled expression of the UL82 homologs in a range of cell types, and permitted investigation of their abilities to direct prolonged gene expression from quiescent genomes. The results show that all UL82 homologs activate gene expression, and that neither host cell type nor promoter target sequence has major effects on these activities. Surprisingly, the UL82 proteins specified by non-human primate cytomegaloviruses, unlike pp71, did not direct long term expression from quiescent HSV-1 genomes. In addition, significant differences were observed in the intranuclear localization of the UL82 homologs, and in their effects on Daxx. Strikingly, S82 mediated the release of Daxx from nuclear domain 10 substructures much more rapidly than pp71 or the other proteins tested. All UL82 homologs stimulated the early release of ATRX from nuclear domain 10.

Conclusion

All of the UL82 homolog proteins analysed activated gene expression, but surprising differences in other aspects of their properties were revealed. The results provide new information on early events in infection with cytomegaloviruses.

Human cytomegalovirus protein pp71 displaces the chromatin-associated factor ATRX from nuclear domain 10 at early stages of infection

The human cytomegalovirus (HCMV) tegument protein pp71, encoded by gene UL82, stimulates viral immediate-early (IE) transcription. pp71 interacts with the cellular protein hDaxx at nuclear domain 10 (ND10) sites, resulting in the reversal of hDaxx-mediated repression of viral transcription. We demonstrate that pp71 displaces an hDaxx-binding protein, ATRX, from ND10 prior to any detectable effects on hDaxx itself and that this event contributes to the role of pp71 in alleviating repression. Introduction of pp71 into cells by transfection, infection with a pp71-expressing herpes simplex virus type 1 vector, or by generation of transformed cell lines promoted the rapid relocation of ATRX from ND10 to the nucleoplasm without alteration of hDaxx levels or localization. A pp71 mutant protein unable to interact with hDaxx did not affect the intranuclear distribution of ATRX. Infection with HCMV at a high multiplicity of infection resulted in rapid displacement of ATRX from ND10, the effect being observed maximally by 2 h after adsorption, whereas infection with the UL82-null HCMV mutant ADsubUL82 did not affect ATRX localization even at 7 h postinfection. Cell lines depleted of ATRX by transduction with shRNA-expressing lentiviruses supported increased IE gene expression and virus replication after infection with ADsubUL82, demonstrating that ATRX has a role in repressing IE transcription. The results show that ATRX, in addition to hDaxx, is a component of cellular intrinsic defenses that limit HCMV IE transcription and that displacement of ATRX from ND10 by pp71 is important for the efficient initiation of viral gene expression.

Induction of cellular stress overcomes the requirement of herpes simplex virus type 1 for immediate-early protein ICP0 and reactivates expression from quiescent viral genomes

Herpes simplex virus type 1 (HSV-1) mutants impaired in the activities of the structural protein VP16 and the immediate-early (IE) proteins ICP0 and ICP4 establish a quiescent infection in human fibroblasts, with most cells retaining an inactive, repressed viral genome for sustained periods in culture. To date, the quiescent state has been considered stable, since it has been reversed only by provision of herpesviral proteins, such as ICP0, not by alteration of the cell physiological state. We report that the interaction of HSV-1 with human fibroblasts can be altered significantly by transient treatment of cultures with sodium arsenite, an inducer of heat shock and oxidative stress, or gramicidin D, a toxin that selectively permeabilizes cell membranes, prior to infection. These regimens stimulated gene expression from IE-deficient HSV-1 mutants in a promoter sequence-independent manner and also overcame the replication defect of ICP0-null mutants. Reactivation of gene expression from quiescent HSV-1 genomes and the resumption of virus replication were observed following addition of arsenite or gramicidin D to cultures. Both agents induced reorganization of nuclear domain 10 structures, the sites of quiescent genomes, but appeared to do so through different mechanisms. The results demonstrate that the physiological state of the cell is important in determining the outcome of infection with IE-deficient HSV-1 and show novel methods for reactivating quiescent HSV-1 in fibroblasts with a high efficiency.

Histone modifications associated with herpes simplex virus type 1 genomes during quiescence and following ICP0-mediated de-repression

In the current study, it was shown that repressed virus genomes in quiescently infected MRC5 cells adopt a repressed histone-associated structure marked by the enrichment of deacetylated histones at a wide variety of herpes simplex virus type 1 (HSV-1) promoters. In addition, it was shown that genome de-repression, mediated by HSV-2 superinfection or delivery of ICP0 using a recombinant adenovirus vector, resulted in the enrichment of acetylated histones on HSV DNA. These data indicate that ICP0-mediated genome de-repression is intimately linked to enrichment of acetylated histones at virus promoters. The fold change in association of pan-acetylated histone H3 following Ad.TRE.ICP0-mediated de-repression consistently revealed promoter-specific variation, with the highest fold changes (>50-fold) being observed at the latency-associated transcript promoter and enhancer regions. Chromatin immunoprecipitation analyses using an antibody specific to the C terminus of histone H3 as a surrogate measure of nucleosome occupancy revealed little variability in the total loading of histone H3 at the various HSV promoters. This observation suggests that acetylation of histone H3 in response to ICP0 expression is not uniformly targeted across the HSV-1 genome during ICP0-mediated de-repression.

Reactivation of expression from quiescent herpes simplex virus type 1 genomes in the absence of immediate-early protein ICP0

Model systems have previously been developed in which herpes simplex virus (HSV) is retained in human fibroblasts in a nonreplicating state known as quiescence. The HSV type 1 (HSV-1) immediate-early (IE) protein ICP0, an important activator of gene expression, reactivates the quiescent genome and promotes the resumption of virus replication. Previous studies reported that infection with ICP0-null HSV-1 mutants fails to reactivate quiescent HSV, even when the mutant itself undergoes productive replication, leading to the hypothesis that quiescent genomes exist in a silent configuration in which they are shielded from trans-acting factors. I reinvestigated these findings, using HSV-1 mutants with lesions in the transcription activators VP16, ICP0, and ICP4 to establish quiescent infection at high efficiency. Superinfection with ICP0-null HSV-1 mutants at a low multiplicity of infection (MOI), so that individual plaques were formed, reactivated expression from the quiescent genome, demonstrating that the requirement for ICP0 is not absolute. The previously reported failure to observe reactivation by ICP0-null mutants was shown to be a consequence of either a low initial MOI or a high superinfecting MOI. Competition between viral genomes at the level of gene expression and virus replication, especially when ICP0 was absent, was demonstrated during reactivation and also during normal infection of human fibroblasts. The results show that the multiplicity-dependent phenotype of ICP0-null mutants limits the efficiency of reactivation at low MOIs and that competition between genomes occurs at high MOIs. The conclusion that quiescent HSV genomes are extensively silenced and intrinsically insensitive to trans-acting factors must be reevaluated.

Betaherpesvirus-conserved cytomegalovirus tegument protein ppUL32 (pp150) controls cytoplasmic events during virion maturation

The UL32 gene of human cytomegalovirus (CMV) encodes a prominent betaherpesvirus-conserved virion tegument protein, called pp150 (basic phosphoprotein/ppUL32), that accumulates within a cytoplasmic inclusion adjacent to the nucleus at late times during infection. Using a UL32 deletion mutant (DeltaUL32-BAC) (where BAC is bacterial artificial chromosome), we demonstrate that pp150 is critical for virion maturation in the cytoplasmic compartment. Cotransfection of a pp150 expression plasmid with DeltaUL32-BAC DNA led to complementation of the replication defect with focus formation due to secondary spread. Deletion of the amino terminus of pp150 or disruption of the betaherpesvirus conserved regions, CR1 and CR2, revealed these regions to be critical for replication. In contrast, deletion of the carboxyl terminus only partially compromised maturation while disruption of glycosylation sites had no effect. An African green monkey CMV UL32 homolog complemented DeltaUL32-BAC replication but murine CMV M32 failed to complement, consistent with evolutionary divergence of rodent and primate cytomegaloviruses. Infection with DeltaUL32-BAC showed normal expression of all kinetic classes of viral genes and replication of viral DNA, with accumulation of viral DNA-containing particles in the cytoplasm; however, mutant virus did not spread to adjacent cells. In contrast to this block in virion infectivity, cell-to-cell transfer of pp65-containing particles was observed, suggesting that release of dense bodies continued in the absence of pp150. These observations demonstrate that pp150 is critical for virion egress, possibly at the stage of final envelopment.

Human cytomegalovirus (HCMV) UL82 gene product (pp71) relieves hDaxx-mediated repression of HCMV replication

This study examines the role of the cellular protein hDaxx in controlling human cytomegalovirus (HCMV) immediate-early (IE) gene expression and viral replication. Using permissive cell lines that either overexpress hDaxx or are depleted of hDaxx expression by the use of short hairpin RNA, we demonstrate that hDaxx functions as a repressor of HCMV IE gene expression and replication. In addition, we demonstrate that the impaired growth phenotype associated with the UL82 (pp71) deletion mutant is abolished when hDaxx knockdown cells are infected, suggesting that pp71 functions to relieve hDaxx-mediated repression during HCMV infection.

Role of the cellular protein hDaxx in human cytomegalovirus immediate-early gene expression

Human cytomegalovirus (HCMV) immediate-early (IE) transcription is stimulated by virion phosphoprotein pp71, the product of gene UL82. It has previously been shown that pp71 interacts with the cellular protein hDaxx and, in the studies presented here, the significance of this interaction was investigated for HCMV IE gene expression. In co-transfection experiments, the presence of hDaxx increased the transcriptional response of the HCMV major IE promoter (MIEP) to pp71, but it was not possible to determine whether the effect was due to an interaction between the two proteins or to stimulation of hDaxx synthesis by pp71. The use of small interfering RNA (siRNA) in long- and short-term transfection approaches reduced intracellular hDaxx levels to no more than 3 % of normal. Infection of hDaxx-depleted cells with herpes simplex virus recombinants containing the HCMV MIEP revealed significantly greater promoter activity when hDaxx levels were minimal. Similarly, reducing intracellular hDaxx amounts resulted in greater IE gene expression during infection with an HCMV mutant lacking pp71, but had no effect on IE transcription during infection with wild-type HCMV. The results suggest that hDaxx is not important as a positive-acting factor for the stimulation of HCMV IE transcription by pp71. Instead, it appears that hDaxx acts as a repressor of IE gene expression, and it is proposed here that the interaction of pp71 with hDaxx is important to relieve repression and permit efficient initiation of productive replication.

Human cytomegalovirus protein pp71 disrupts major histocompatibility complex class I cell surface expression

The human cytomegalovirus tegument protein pp71 is the product of the UL82 gene. Roles for pp71 in stimulating gene transcription, increasing infectivity of viral DNA, and the degradation of retinoblastoma family proteins have been described. Here we report a novel function for pp71 in limiting accumulation of cell surface major histocompatibility complex (MHC) class I complexes. MHC molecules were analyzed in glioblastoma cells exposed to a replication-defective adenovirus expressing UL82 (Adpp71) or after transient transfection of the UL82 gene. Accumulation of cell surface MHC class I levels diminished in a specific and dose-dependent manner after exposure to Adpp71 but not after exposure to an adenovirus expressing beta-galactosidase (Adbeta gal). UL82 expression did not interfere with accumulation of either MHC class I heavy-chain transcript or protein, nor did UL82 expression correlate with markers of apoptosis. Rather, UL82 expression correlated with an increased proportion of MHC class I molecules exhibiting sensitivity to endoglycosidase H treatment. Finally, we show that, in cells infected with recombinant virus strain missing all of the unique short region MHC class I evasion genes, disruption of UL82 expression by short, interfering RNAs led to increased accumulation of cell surface MHC class I complexes. These findings support a novel role for HCMV pp71 in disruption of the MHC class I antigen presentation pathway.

New genes from old: redeployment of dUTPase by herpesviruses

Published work (D. J. McGeoch, Nucleic Acids Res. 18:4105-4110, 1990; J. E. McGeehan, N. W. Depledge, and D. J. McGeoch, Curr. Protein Peptide Sci. 2:325-333, 2001) has indicated that evolution of dUTPase in the class of herpesviruses that infect mammals and birds involved capture of a host gene followed by a duplication event that resulted in a coding region comprising two fused dUTPase domains. Some of the conserved residues required for enzyme activity were then lost, resulting in a dUTPase containing a single active site with different elements contributed by each half of the protein. Further conserved residues were lost in one subfamily (the Betaherpesvirinae), yielding a protein that is related to herpesvirus dUTPases but has a different and as yet unrecognized function. Evidence from sequence similarities and structural predictions now indicates that several additional genes were derived from the herpesvirus dUTPase gene, probably by duplication. These are UL31, UL82, UL83, and UL84 in human cytomegalovirus (and counterparts in other members of the Betaherpesvirinae) and ORF10 and ORF11 in human herpesvirus 8 (and counterparts in other members of the Gammaherpesvirinae). The findings clarify the evolutionary history of these genes and provide novel insights for structural and functional studies.

Interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication

The human cytomegalovirus UL82-encoded pp71 protein is required for efficient virus replication and immediate-early gene expression when cells are infected at a low multiplicity. Functions attributed to pp71 include the ability to enhance the infectivity of viral DNA, bind to and target hypophosphorylated Rb family member proteins for degradation, drive quiescent cells into the cell cycle, and bind to the cellular protein hDaxx. Using UL82 mutant viruses, we demonstrate that the LXCXD motif within pp71 is not necessary for efficient virus replication in fibroblasts, suggesting that pp71's ability to degrade hypophosphorylated Rb family members and induce quiescent cells into the cell cycle is not responsible for the growth defect associated with a UL82 deletion mutant. However, UL82 mutants that cannot bind to hDaxx are unable to induce immediate-early gene expression and are severely attenuated for viral replication. These results indicate that the interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication.

Human cytomegalovirus tegument protein pp71 directs long-term gene expression from quiescent herpes simplex virus genomes

The human cytomegalovirus tegument protein pp71 is important for transactivation of immediate-early (IE) gene expression and for the efficient initiation of virus replication. We have analyzed the properties of pp71 by assaying its effects on gene expression from the genome of in1312, a herpes simplex virus type 1 (HSV-1) mutant devoid of functional VP16, ICP0, and ICP4. Upon infection of human fibroblasts, in1312-derived viruses are repressed and retained in a quiescent state, but the presence of pp71 prevented the quiescent state from being attained. Reporter gene cassettes cloned into the in1312 genome, in addition to the endogenous IE promoters, remained active for at least 12 days postinfection, and infected cells were viable and morphologically normal. Cells expressing pp71 remained responsive to the HSV-1 transactivating factors VP16 and ICP4 and to trichostatin A. The C-terminal 61 amino acids, but not the LACSD motif, were required for pp71 activity. In addition to preventing attainment of quiescence, pp71 was able to disrupt the quiescent state of in1312 derivatives and promote the resumption of viral gene expression after a lag of approximately 3 days. The results extend the functional analysis of pp71 and suggest a degree of similarity with the HSV-1 IE protein ICP0. The ability to provoke slow reactivation of quiescent genomes, in conjunction with cell survival, represents a novel property for a viral structural protein.

Requirement of Sur2 for efficient replication of mouse adenovirus type 1

Mouse adenovirus type 1 (MAV-1) early region 1A (E1A) encodes a virulence gene in viral infection of mice. To broaden our understanding of the functions of E1A in MAV-1 pathogenesis, an unbiased experimental approach, glutathione S-transferase (GST) pulldown, was used to screen for cellular proteins that interact with E1A protein. We identified mouse Sur2, a subunit of Mediator complex, as a protein that binds to MAV-1 E1A. The interaction between Sur2 and MAV-1 E1A was confirmed in virus-infected cells. Conserved region 3 (CR3) of MAV-1 E1A was mapped as the region required for Sur2-E1A interaction, as is the case for human adenovirus E1A. Although it has been proposed that human adenovirus E1A recruits the Mediator complex to transactivate transcription of viral early genes, Sur2 function in adenovirus replication has not been directly tested previously. Studies on the functions of Sur2 with mouse embryonic fibroblasts (MEFs) showed that there was a multiplicity-dependent growth defect of MAV-1 in Sur2(-/-) MEFs compared to Sur2(+/+) MEFs. Comparison of the viral DNA and viral mRNA levels in Sur2(+/+) and Sur2(-/-) MEFs confirmed that Sur2 was important for efficient viral replication. The viral replication defects in Sur2(-/-) MEFs appeared to be due at least in part to a defect in viral early gene transcription.

Molecular, biological, and in vivo characterization of the guinea pig cytomegalovirus (CMV) homologs of the human CMV matrix proteins pp71 (UL82) and pp65 (UL83)

We recently identified the genes encoding the guinea pig cytomegalovirus (GPCMV) homologs of the upper and lower matrix proteins of human CMV, pp71 (UL82) and pp65 (UL83), which we designated GP82 and GP83, respectively. Transient-expression studies with a GP82 plasmid demonstrated that the encoded protein targets the nucleus and that the infectivity and plaquing efficiency of cotransfected GPCMV viral DNA was enhanced by GP82. The transactivation function of GP82 was not limited to GPCMV, but was also observed for a heterologous virus, herpes simplex virus type 1 (HSV-1). This was confirmed by its ability to complement the growth of an HSV-1 VP16 transactivation-defective mutant virus in an HSV viral DNA cotransfection assay. Study of a GP82 "knockout" virus (and its attendant rescuant), generated on a GPCMV bacterial artificial chromosome construct, confirmed the essential nature of the gene. Conventional homologous recombination was used to generate a GP83 mutant to examine the role of GP83 in the viral life cycle. Comparison of the one-step growth kinetics of the GP83 mutant (vAM409) and wild-type GPCMV indicated that GP83 protein is not required for viral replication in tissue culture. The role of GP83 in vivo was examined by comparing the pathogenesis of wild-type GPCMV, vAM409, and a control virus, vAM403, in guinea pigs. The vAM409 mutant was significantly attenuated for dissemination in immunocompromised strain 2 guinea pigs, suggesting that the GP83 protein is essential for full pathogenicity in vivo.

Role of the human cytomegalovirus major immediate-early promoter's 19-base-pair-repeat cyclic AMP-response element in acutely infected cells

Prior studies have suggested a role of the five copies of the 19-bp-repeat cyclic AMP (cAMP)-response element (CRE) in major immediate-early (MIE) promoter activation, the rate-limiting step in human cytomegalovirus (HCMV) replication. We used two different HCMV genome modification strategies to test this hypothesis in acutely infected cells. We report the following: (i) the CREs do not govern basal levels of MIE promoter activity at a high or low multiplicity of infection (MOI) in human foreskin fibroblast (HFF)- or NTera2-derived neuronal cells; (ii) serum and virion components markedly increase MIE promoter-dependent transcription at a low multiplicity of infection (MOI), but this increase is not mediated by the CREs; (iii) forskolin stimulation of the cAMP signaling pathway induces a two- to threefold increase in MIE RNA levels in a CRE-specific manner at a low MOI in both HFF- and NTera2-derived neuronal cells; and (iv) the CREs do not regulate basal levels of HCMV DNA replication at a high or low MOI in HFF. Their presence does impart a forskolin-induced increase in viral DNA replication at a low MOI but only when basal levels of MIE promoter activity are experimentally diminished. In conclusion, the 19-bp-repeat CREs add to the robust MIE promoter activity that occurs in the acutely infected stimulated cells, although the CREs' greater role may be in other settings.

Daxx-mediated accumulation of human cytomegalovirus tegument protein pp71 at ND10 facilitates initiation of viral infection at these nuclear domains

Human cytomegalovirus (HCMV) starts immediate-early transcription at nuclear domains 10 (ND10), forming a highly dynamic immediate transcript environment at this nuclear site. The reason for this spatial correlation remains enigmatic, and the mechanism for induction of transcription at ND10 is unknown. We investigated whether tegument-based transactivators are involved in the specific intranuclear location of HCMV. Here, we demonstrate that the HCMV transactivator tegument protein pp71 accumulates at ND10 before the production of immediate-early proteins. Intracellular trafficking of pp71 is facilitated through binding to a coiled-coil region of Daxx. The C-terminal domain of Daxx then interacts with SUMO-modified PML, resulting in the deposition of pp71 at ND10. In Daxx-deficient cells, pp71 does not accumulate at ND10, proving in vivo the necessity of Daxx for pp71 deposition. Also, HCMV forms immediate transcript environments at sites other than ND10 in Daxx-deficient cells, and so does the HCMV pp71 knockout mutant UL82(-/-) in normal cells. This result strongly suggests that pp71 and Daxx are essential for HCMV transcription at ND10. Lack of Daxx had the effect of reducing the infection rate. We conclude that the tegument transactivator pp71 facilitates viral genome deposition and transcription at ND10, possibly priming HCMV for more efficient productive infection.

Activity and intracellular localization of the human cytomegalovirus protein pp71

The human cytomegalovirus (HCMV) tegument phosphoprotein pp71 activates viral immediate early (IE) transcription and thus has a role in initiating lytic infection. Protein pp71 stimulates expression from a range of promoters in a sequence-independent manner, and in this respect behaves similarly to the herpes simplex virus type 1 (HSV-1) IE protein ICP0. The intracellular localization of pp71 was investigated after its expression from transfected plasmids or from HSV-1 mutants constructed to produce pp71 transiently. The protein colocalized with the cell promyelocytic leukaemia (PML) protein at nuclear domain 10 (ND10) structures but, unlike ICP0, pp71 did not induce disruption of ND10. The activity of pp71 in mouse sensory neurons in vivo was investigated after co-inoculation of animals with pairs of HSV-1 mutants, one expressing pp71 and the second containing the E. coli lacZ gene controlled by various promoters. In this system, pp71 stimulated beta-galactosidase expression from a range of viral IE promoters when mice were analysed at 4 days postinoculation. At later times, expression of pp71 resulted in a reduction in numbers of neurons containing beta-galactosidase, indicating cytotoxicity or promoter shutoff. The HSV-1 latency-active promoter was not responsive to pp71, demonstrating specificity in the activity of the protein. Pp71 was as active in mice lacking both copies of the PML gene (PML-/-) as in control animals, and in PML-/- fibroblasts pp71 stimulated gene expression as effectively as in other cell types. Therefore, neither the PML protein nor the normal ND10 structure is necessary for pp71 to stimulate gene expression.

Functional interaction between the pp71 protein of human cytomegalovirus and the PML-interacting protein human Daxx

The tegument protein pp71 (UL82) of human cytomegalovirus (HCMV) has previously been shown to transactivate the major immediate-early enhancer-promoter of HCMV. Furthermore, this protein is able to enhance the infectivity of viral DNA and to accelerate the infection cycle, suggesting an important regulatory function during viral replication. To gain insight into the underlying mechanisms that are used by pp71 to exert these pleiotropic effects, we sought for cellular factors interacting with pp71 in a yeast two-hybrid screen. Here, we report the isolation of the human Daxx (hDaxx) protein as a specific interaction partner of HCMV pp71. hDaxx, which was initially described as an adapter protein involved in apoptosis regulation, has recently been identified as a nuclear protein that interacts and colocalizes with PML in the nuclear domain ND10. In order to assess whether pp71 can also be detected in ND10 structures, a vector expressing pp71 in fusion with the green fluorescent protein was used for transfection of human fibroblasts. This revealed a colocalization of pp71 with the ND10 proteins PML and Sp100. In addition, cotransfection of a hDaxx expression vector resulted in an enhanced recruitment of pp71 to ND10. Targeting of pp71 to nuclear dots could also be observed in infected human fibroblasts in the absence of de novo viral protein synthesis. Moreover, cotransfection experiments revealed that pp71-mediated transactivation of the major immediate-early enhancer-promoter was synergistically enhanced in the presence of hDaxx. These results suggest an important role of hDaxx for pp71 protein function.

The human cytomegalovirus UL35 gene encodes two proteins with different functions

The human cytomegalovirus (HCMV) virion is a complex structure that contains at least 30 proteins, many of which have been identified. We determined that the HCMV UL35 gene encodes two proteins, including a previously unidentified virion protein. A 22-kDa phosphoprotein (ppUL35(A)) was translated from a 1.2-kb UL35 transcript by 4 h postinfection; a second phosphoprotein of 75 kDa (ppUL35) was translated from a 2.2-kb transcript predominantly late in infection. The 22-kDa protein localized to the nucleus, while the 75-kDa protein localized to the juxtanuclear compartment and was packaged into virion particles. The 22-kDa protein was identical to the COOH-terminal end of the 75-kDa protein but was not found in virions, thus defining the NH(2)-terminal portion of the 75-kDa protein as essential for packaging. Expression of the 22-kDa protein inhibited activation of the major immediate-early promoter by ppUL82 (pp71), suggesting that the UL35 22-kDa protein may modulate expression of the major immediate-early gene.

Requirement of multiple cis-acting elements in the human cytomegalovirus major immediate-early distal enhancer for viral gene expression and replication

We have shown previously that the human cytomegalovirus (HCMV) major immediate-early (MIE) distal enhancer is needed for MIE promoter-dependent transcription and viral replication at low multiplicities of infection (MOI). To understand how this region works, we constructed and analyzed a series of HCMVs with various distal enhancer mutations. We show that the distal enhancer is composed of at least two parts that function independently to coordinately activate MIE promoter-dependent transcription and viral replication. One such part is contained in a 47-bp segment that has consensus binding sites for CREB/ATF, SP1, and YY1. At low MOI, these working parts likely function in cis to directly activate MIE gene expression, thus allowing viral replication to ensue. Three findings support the view that these working parts are likely cis-acting elements. (i) Deletion of either part of a bisegmented distal enhancer only slightly alters MIE gene transcription and viral replication. (ii) Reversing the distal enhancer's orientation largely preserves MIE gene transcription and viral replication. (iii) Placement of stop codons at -300 or -345 in all reading frames does not impair MIE gene transcription and viral replication. Lastly, we show that these working parts are dispensable at high MOI, partly because of compensatory stimulation of MIE promoter activity and viral replication that is induced by a virion-associated component(s) present at a high viral particle/cell ratio. We conclude that the distal enhancer is a complex multicomponent cis-acting region that is required to augment both MIE promoter-dependent transcription and HCMV replication.

Human cytomegalovirus binding to heparan sulfate proteoglycans on the cell surface and/or entry stimulates the expression of human leukocyte antigen class I

Human cytomegalovirus (HCMV) is known to down-regulate the expression of human leukocyte antigen (HLA) class I, the process of which involves a subset of virus genes. Infection of human foreskin fibroblast (HFF) cells with UV-inactivated HCMV (UV-HCMV), however, resulted in an increase in HLA class I presentation on the cell surface in the absence of HCMV gene expression. Heparin, which inhibits the interaction of virus particles with cell surface heparan sulfate proteoglycans (HSPGs), blocked the effect of UV-HCMV on HLA class I expression. Pretreatment of cells with heparinase I decreased in a dose-dependent manner the effect of UV-HCMV on HLA class I expression enhancement. Sodium chlorate, which is known to inhibit the sulfation of HSPGs, gave a similar result. Pretreatment of UV-HCMV with trypsin or monoclonal antibody reactive with the envelope glycoprotein gB reduced the increase in HLA class I expression on the HFF cell surface by UV-HCMV. RT-PCR analysis demonstrated that the increase in HLA class I presentation on the HFF cell surface was due to an increase in HLA class I transcription. Thus, binding of HCMV particles to cell surface HSPGs appears to be required for the stimulation of HLA class I expression. It is also possible that virus entry, in addition to binding to HSPGs, may be involved in the stimulation of HLA class I expression, since the UV-HCMV entered the cells and all treatments to block virus binding to HSPGs would necessarily prevent virus entry.

Activation of interferon response factor-3 in human cells infected with herpes simplex virus type 1 or human cytomegalovirus

Activation of cellular interferon-stimulated genes (ISGs) after infection with herpes simplex virus type 1 (HSV-1) or human cytomegalovirus (HCMV) was investigated. The level of ISG54-specific RNA in human fetal lung (HFL) or human foreskin (BJ) fibroblasts increased substantially after infection with either virus in the presence of cycloheximide. HSV-1 particles lacking glycoprotein D or glycoprotein H failed to induce ISG54-specific RNA synthesis, demonstrating that entry of virus particles rather than binding of virions to the cell surface was required for the effect. A DNA-binding complex that recognized an interferon-responsive sequence motif was induced upon infection with HSV-1 or HCMV in the presence of cycloheximide, and the complex was shown to contain the cell proteins interferon response factor 3 (IRF-3) and CREB-binding protein. IRF-3 was modified after infection with HSV-1 or HCMV to a form of lower electrophoretic mobility, consistent with phosphorylation. De novo transcription of viral or cellular genes was not required for the activation of IRF-3, since the effect was not sensitive to inhibition by actinomycin D. Infection of HFL fibroblasts with HSV-1 under conditions in which viral replication proceeded normally resulted in severely reduced levels of the IRF-3-containing complex, defining the activation of IRF-3 as a target for viral interference with ISG induction. In BJ fibroblasts, however, significant activation of IRF-3 was detected even when the viral gene expression program progressed to later stages, demonstrating that the degree of inhibition of the response was dependent on host cell type. As a consequence of IRF-3 activation, endogenous interferon was released from BJ cells and was capable of triggering the appropriate signal transduction pathway in both infected and uninfected cells. Activation of ISG54-specific RNA synthesis was not detected after infection of human U-373MG glioblastoma cells, showing that the induction of the response by infection is cell type dependent.

Latency associated promoter transgene expression in the central nervous system after stereotaxic delivery of replication-defective HSV-1-based vectors

The herpes simplex virus type 1 (HSV-1) latency associated promoter (LAP) has been shown to sustain long-term reporter gene expression within sensory neurones. Its activity within the CNS is, however, less well understood. In this study we characterise the activity of the LAP after stereotaxic delivery of recombinant HSV-1-based vectors to the brain. Two classes of vectors were utilised in these studies: (1) a replication-defective vector lacking the glycoprotein H and thymidine kinase genes, designated CS1, and (2) a virus mutant severely impaired for immediate-early (IE) gene expression which lacks functional VP16, ICP4 and ICP0 genes, designated in1388. Both vectors contain the LacZ gene under the control of the LAP. Following delivery of either vector to the striatum, beta-gal expression was detected within anatomically related CNS regions distal to the site of injection. At these sites the number of beta-gal-positive cells increased with time and remained stable up to 4 weeks p.i. beta-Gal expression could not be detected at the site of injection after delivery of CS1 but beta-gal expression within neurones located at this site was observed after delivery of in1388, indicating reduced toxicity of this severely disabled virus. Transgene expression decreased dramatically with both vectors at later time-points (>4 weeks after delivery), but PCR analysis demonstrated that viral genomes were stably maintained for up to 180 days following delivery, indicating that the loss of beta-gal-positive neurones was not likely to be due to a loss of vector-transduced cells. Moreover, after delivery of an equivalent virus to the rat striatum in situ hybridisation analysis showed a similar decrease in the number of neurones expressing the endogenous LATs with time. These data indicate that although the HSV-1 LAP can drive the expression of foreign genes in a variety of CNS neurones, in these cells there is a slow down-regulation of the viral promoter which eventually results in the loss of detectable transgene expression.

Coevolutionary networks: a novel approach to understanding the relationships of humans with the infectious agents

Human organism is interpenetrated by the world of microorganisms, from the conception until the death. This interpenetration involves different levels of interactions between the partners including trophic exchanges, bi-directional cell signaling and gene activation, besides genetic and epigenetic phenomena, and tends towards mutual adaptation and coevolution. Since these processes are critical for the survival of individuals and species, they rely on the existence of a complex organization of adaptive systems aiming at two apparently conflicting purposes: the maintenance of the internal coherence of each partner, and a mutually advantageous coexistence and progressive adaptation between them. Humans possess three adaptive systems: the nervous, the endocrine and the immune system, each internally organized into subsystems functionally connected by intraconnections, to maintain the internal coherence of the system. The three adaptive systems aim at the maintenance of the internal coherence of the organism and are functionally linked by interconnections, in such way that what happens to one is immediately sensed by the others. The different communities of infectious agents that live within the organism are also organized into functional networks. The members of each community are linked by intraconnections, represented by the mutual trophic, metabolic and other influences, while the different infectious communities affect each other through interconnections. Furthermore, by means of its adaptive systems, the organism influences and is influenced by the microbial communities through the existence of transconnections. It is proposed that these highly complex and dynamic networks, involving gene exchange and epigenetic phenomena, represent major coevolutionary forces for humans and microorganisms.

Human neuron-committed teratocarcinoma NT2 cell line has abnormal ND10 structures and is poorly infected by herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 stimulates the initiation of lytic infection and reactivation from quiescence in human fibroblast cells. These functions correlate with its ability to localize to and disrupt centromeres and specific subnuclear structures known as ND10, PML nuclear bodies, or promyelocytic oncogenic domains. Since the natural site of herpesvirus latency is in neurons, we investigated the status of ND10 and centromeres in uninfected and infected human cells with neuronal characteristics. We found that NT2 cells, a neuronally committed human teratocarcinoma cell line, have abnormal ND10 characterized by low expression of the major ND10 component PML and no detectable expression of another major ND10 antigen, Sp100. In addition, PML is less extensively modified by the ubiquitin-like protein SUMO-1 in NT2 cells compared to fibroblasts. After treatment with retinoic acid, NT2 cells differentiate into neuron-like hNT cells which express very high levels of both PML and Sp100. Infection of both NT2 and hNT cells by HSV-1 was poor compared to human fibroblasts, and after low-multiplicity infection yields of virus were reduced by 2 to 3 orders of magnitude. ICP0-deficient mutants were also disabled in the neuron-related cell lines, and cells quiescently infected with an ICP0-null virus could be established. These results correlated with less-efficient disruption of ND10 and centromeres induced by ICP0 in NT2 and hNT cells. Furthermore, the ability of ICP0 to activate gene expression in transfection assays in NT2 cells was poor compared to Vero cells. These results suggest that a contributory factor in the reduced HSV-1 replication in the neuron-related cells is inefficient ICP0 function; it is possible that this is pertinent to the establishment of latent infection in neurons in vivo.

UL82 virion protein activates expression of immediate early viral genes in human cytomegalovirus-infected cells

The human cytomegalovirus UL82 gene encodes a protein (pp71) that is localized in the tegument domain of the virus particle. The UL82 gene product is delivered to the nucleus at the time of infection, and it is believed to function in gene activation. We have constructed a human cytomegalovirus mutant, ADsubUL82, that lacks a substantial portion of the UL82 coding region. It was propagated on human diploid fibroblasts expressing the UL82 gene product, and it was possible to produce a mutant virus lacking the UL82 protein by passaging virus stocks for one cycle of growth on normal, noncomplementing fibroblasts. The UL82-deficient mutant displays a multiplicity-dependent growth defect in normal human fibroblasts. The growth of ADsubUL82 is severely restricted at low input multiplicities (0.01-0.1 plaque-forming units per cell), producing a yield that is reduced by a factor of about 10(5) in comparison to wild-type virus. At higher input multiplicities (10 plaque-forming units per cell), ADsubUL82 grew nearly as well as the wild-type virus. By using a human cytomegalovirus gene array, we demonstrated that UL82 functions to facilitate virus mRNA accumulation very early during the human cytomegalovirus replication cycle. The growth phenotype associated with the UL82 mutant seems to result from its inability to efficiently activate human cytomegalovirus immediate early genes.

Activation of cellular interferon-responsive genes after infection of human cells with herpes simplex virus type 1

Previous studies have shown that infection of human fibroblasts with human cytomegalovirus (HCMV) results in activation of cellular interferon-responsive gene expression. We demonstrate here that infection of human fibroblasts with herpes simplex virus type 1 (HSV-1) in the absence of de novo protein synthesis also induces the expression of interferon-responsive genes. Five genes tested (encoding ISG54, IFI56, ISG15, 9-27 and MxA) were activated by infection with HSV-1, although the degree of response varied between the individual genes. HSV-1 was a less efficient inducer than HCMV. The effect was a consequence of binding of the virus particle to the cell surface or of the presence of virion components within the infected cell. Induction was mediated by a pathway other than the mechanism through which interferon-alpha mediates its effects on cellular gene expression.